1. Field of the Invention
The present invention relates to compound semiconductor field-effect transistors, particularly to semiconductor devices that include an n-type high electron mobility transistor and a p-type field-effect transistor formed on the same compound semiconductor substrate.
2. Description of the Related Art
Field-effect transistors with a compound semiconductor layer of a material such as GaAs have high electron mobility and desirable frequency characteristics, and for this reason have been widely used with an n-channel for high-frequency region applications such as in cellular phones. The n-channel FET (Field Effect Transistor) currently in use for the high frequency band is the high electron mobility transistor (HEMT). Its variant pseudomorphic high electron mobility transistor (PHEMT; Pseudomorphic HEMI) is also known that tolerates certain degrees of lattice mismatch in epitaxial growth, and thus realizes even higher electron mobility. JPHEMTs (Junction Pseudomorphic HEMTs) that form a PN junction at a gate portion are also known (see, for example, JP-A-11-150264). In JPHEMTs, a large positive voltage is applied to the gate to reduce the carrier depleted region formed in a channel layer, and thereby reduce the parasitic resistance component of the channel layer.
With the movement toward high performance n-channel FETs, there is an increasing demand for higher integration, which requires the development of complementary elements using a compound semiconductor. Ion implantation is a common technique to simultaneously form n-channel and p-channel FETs on a compound semiconductor. In this technique, an n-channel forming region and a p-channel forming region are formed by selectively injecting a p-type dopant and an n-type dopant into the same substrate. However, the ion implantation technique requires high-temperature annealing of at least 800° C. after the ion implantation, in order to activate the injected dopants.
JP-A-61-67275 describes a method of simultaneously forming an n-channel FET and a p-channel FET on a compound semiconductor. According to this publication, an n-channel-type heterojunction field-effect transistor using a two-dimensional electron gas as the carrier, and a p-channel-type heterojunction field-effect transistor using a two-dimensional hole gas as the carrier are formed on a compound semiconductor substrate of GaAs. The n-channel-type heterojunction field-effect transistor includes a non-doped GaAs layer, a non-doped AlGaAs layer, an n-type impurity doped AlGaAs layer, and an n-type impurity doped GaAs layer. Each layer is laminated using an epitaxial growth method. The n-type impurity doped AlGaAs layer has a gate electrode of aluminum or titanium/platinum/gold. The n-type impurity doped GaAs layer has source and drain electrodes of gold-germanium/gold. The p-channel-type heterojunction field-effect transistor includes a non-doped GaAs layer, a non-doped AlGaAs layer, a p-type impurity doped AlGaAs layer, and a p-type impurity doped GaAs layer laminated on the laminate structure using an epitaxial growth method. The p-type impurity doped AlGaAs layer has a gate electrode of Al or titanium/platinum/gold. Source and drain electrodes of gold/zinc/gold are formed on the p-type impurity doped GaAs layer.